What many industries need is a time-temperature indicator for learning what is going on in the field. For example, in the frozen food industry, a shipper would like to know if the shipments are being handled properly and if not, where they are being mishandled. To do this the shipper would need reliable time-temperature information available to him.
One severe limitation of prior art time-temperature indicators such as those disclosed in U.S. Pat. No. 3,414,415 by Broad and U.S. Pat. No. 3,479,877 by Allen et al. is that they require on the spot trained observers to note, interpret and conclude what the indicator says. If the observer removes the indicator, it will continue to record change. If the observer forgets to read it promptly after removal, the information will probably be inaccurate. If the observer knows what the indicator should read, he can furnish the answers he thinks the shipper will like.
It is an object of this invention to provide an indicator in which the time-temperature recordation can be stopped at any desired point in time, and which does not require an on the spot trained observer to interpret.